• 


U.  S.  Department  of  Agriculture,  Forest  Service 

FOREST  PRODUCTS  LABORATORY 

In  cooperation  with  the  University  of  Wisconsin 
MADISON,  WISCONSIN 


PULPING  EASTERN  HEMLOCK 

BY  THE  SULPHITE  PROCESS 

1.     The  effect  of  varying  the  time  and  the 
temperature  of  impregnation 


By  W.  H.  MONSOON. 


Associate  Chemist  in  Forest  Products 

and 

G.  H.  CHIDESTER 

Assistant  Engineer  in  Forest  Products 


f?.  W 


Published   in 


PAPER  TRADE  JOURNAL 
November  15,   1928 


PULPING  EASTERN  HEMLOCK  3Y  THE  SULPHITE  PROCESS 
I. — THE  EE         ARYING  THE  TILS   AN!  THE 
TEMPERATURE  OE  IMPREGNATI 


By 
'.".  H.  Konsson 
Associate  Chemist  in  Eorest* Products 

and 

G.  H.  Chidester 
Assistant  Engineer  in  Forest  Products 


Introduction 

Although  the  consumption  of  eastern  hemlock  by 
sulphite  mills  in  the  Lake  States  region  has  increased  during 
the  past  few  years,  mainly  as  a  result  of  the  scarcity  and 
the  high  price  of  domestic  spruce,  no  literature  bearing  on 
the  question  of  optimum  cooking  conditions  for  the  species 
has  been  made  available.   Accordingly  experimental  studies 
in  the  pulping  of  eastern  hemlock  by  the  sulphite  process 
were  undertaken  at  the  U.  S.  Eorest  Products  Laboratory.   The 
penetration  period  was  selected  as  the  first  subject  for 
investigation  because  it  is  now  recognized  as  a  separate  and 
extremely  important  portion  of  the  total  pulping  cycle.   The 
immediate  objective  of  the  studies  was  to  determine  if  possible 
the  optimum  conditions  for  penetration  of  the  chips  as  shown 
by  the  yield  of  screened  pulp,  the  amount  of  screenings,  and 
the  strength  qualities  and  bleach  requirements  of  the  pulps 
produced.   This  paper,  the  first  of  the  series  presenting  the 
information  derived,  sets  forth  the  results  obtained  in  a 
group  of  semi-commercial-scale  pulping  experiments  in  which 
either  the  time  or  the  temperature  of  the  penetration  period 
was  varied,  other  factors  being  held  constant. 

Previous  Tork 

During  the  past  few  years  the  Laboratory  has  made 
many  pulping  tests  on  eastern  hemlock  and  similar  species,  but 
the  work  has  been  of  an  exploratory  nature  and  as  a  result  no 
particular  effort  was  made  to  arrive  at  optimum  pulpi 


R883 


Digitized  by  the  Internet  Archive 

in  2013 


http://archive.org/details/easternheOOfore 


conditions.   On  the  other  hand,  the  results  so  obtained  have 
been  valuable  as  a  guide  in  this  study. 


Characteristics  of  Wood 


The  species  of  the  wood  employed  in  the  experiments 
was  identified  as  T supra  canadensis.   The  pulp  wood  used  for 
the  tests  consisted  of  tops  cut  from  trees  grown  in  north- 
eastern Wisconsin.   In  order  to  obtain  an  idea  of  the  uni- 
formity of  the  wood  as  well  as  to  record  certain  physical 
properties,  specific  gravity  tests  were  made  on  disks  cut 
from  a  number  of  the  logs.   The  specific  gravity  values  are 
expressed  as  oven-dry  weight  of  green  volume.   The  samples, 
which  were  taken  at  random,  represented  approximately  30  per 
cent  of  xhe  total  logs  used.   The  age  of  each  specimen  was 
determined  by  counting  the  annual  rings.   The  average  diameter 
of  the  disk  inside  the  bark  was  also  determined.   A  value  for 
average  growth  rate  was  obtained  by  dividing  the  number  of 
annual  rings  by  the  radius  of  the  bolt.   All  these  values  are 
given  in  Table  1. 

Specific  Gravity  Varies  Widely 

The  tabulation  makes  evident  the  fact  that  the  spe- 
cific gravity  of  the  wood,  which  influences  considerably  the 
penetration  of  the  chips  by  the  acid  cooking  liquor,  varied 
widely.   For  spruce  and  balsam  fir  it  has  been  shown!  that 
there  is  a  relationship  between  the  average  growth  rate  and 
the  specific  gravity  of  the  bolts.   This  relationship  did  not 
hold  for  the  eastern  hemlock  examined. 

In  order  to  obtain  as  much  uniformity  as  possible  in 
the  material  for  the  various  cooks,  the  chips  forming  the 
total  supply  were  thoroughly  mixed  before  any  of  the  experimen- 
tal cooks  were  made. 


Temoerature  and  Time  Schedules 


The  temperature  and  time  schedule,  as  pointed  out  in 
the  pulping  of  sprucci.  and  of  hardwoods,  3.  has  been  shown  to  be 

—   Pulo  and  Paper  Ma  ;.  of  Canada,  International  Number,  139-149 

(Feb.,  1933). 

2  Paper  Trade  J.  82,  No.  9,  82-G4  (1926). 

1  Paper  Tra.de  J.  86,  No.  17,  59-62  (1923). 

R883  -2- 


Table  1. — Physical  properties  of  eastern  hemlock  pulp  wood 


D  Lsk 

No. 

:    )  c  i  i  i  c 
:   -avity 

:  (o\ren-dry 
:  volume ) 

:      .t 
:per  cubic 
:   foot 
: (oven-dry) 

:   Age 
; ( annual 
:  rings) 

:   Average 
:   dia?neter 

: (barked  and 
:  oven-dry) 

:  Annual  rin 
:   per  inch 

:  Pounds 

:  Years 

:    Inches 

1 

:   0.386 

:   24.1 

:   100 

:     6.37 

:    35.40 

2 

:    .448 

:   28.0 

:   168 

:    5 .  00 

:    56.00 

3 

!    .497 

:   31.1 

:   130 

:    3.56 

:    39.70 

4 

:   .419  ] 

:    26.2 

:   46 

:     5.87 

:    15.70 

5  : 

.429   ! 

25.8 

:   188 

:    3.87 

:    54.80 

6 

.416 

26.0 

:   159 

:    7.31 

;     43 . 50 

7  ; 

.452   : 

28.2 

:   157 

:    7.56 

:    41.60 

8  : 

.406   1 

25.4 

98 

:    8.87 

28.60 

9  : 

.479   : 

30.0    : 

212   ! 

4.93 

86.00 

10  : 

.573   ; 

35.8 

150 

5 .  00 

60.00 

11  : 

.424   ! 

°5»5   : 

oc 

5.62    ! 

:     25.80 

13  : 

.491   : 

30.7    • 

110   ] 

4.37    ; 

50.10 

13  i 

I-?  /n  r\ 

22.9 

49 

\             .50   ! 

:    15.06 

14 

.473   : 

^J  \J  m    «-^ 

181 

:    7.00 

;     51.30 

15  : 

.405   : 

25.4 

:  105  ; 

32.90 

16  ! 

.415  : 

25.9 

!   135 

\          7.12   : 

1     57.90 

17  ! 

.401  : 

25.0 

!   165 

:    7.50   : 

!     44.00 

Aver- 
age ' 

:  0.440  : 

27.5 

!  130  : 

3.54 

!     41.00 

R383 


-3- 


one  of  the  major  factors  in  the  pulping  of  any  species  by  the 

sulphite  process.   In  this  series  of  experiments  both  the  time 

and  the  temperature  of  the  penetration  period  were  varied  in 
turn,  one  of  the  two  factors  always  remaining  constant. 

The  pulping  schedules  followed  from  the  end  of  the 
penetration  period  to  the  maximum  temperature,  148  degrees 
C,  were  all  of  seven  hours  duration  and  of  the  same  type, 
namely,  the  so-called  "straight-line  rise."   The  temperature 
at  the  beginning  of  the  pulping  period  varied,  depending  upon 
the  highest  temperature  of  the  penetration  period.   Three  such 
high  temperatures  were  used,  110  degrees,  115  degrees,  and  120 
degrees  C.   The  time  required  to  reach  these  temperatures  was 
varied  as  follows:   1.5,  2.0,  2.5,  3.5,  and  5  hours  to  110 
degress  C. ,  1.5,  2.0,  and  2.5  hours  to  115  degrees  C. ,  and  the 
same  to  120  degrees  C. 

The  effects  of  the  variations  in  procedure  just 
described  were  recorded  by  determining  the  yield  of  screened 
pulp  and  screenings,  the  bleachability,  and  the  strength 
development  of  the  pulps  produced. 

Experimental  Equipment  and  Procedure 

The  experimental  cooks  were  all  made  in  the  Labora- 
tory's semi-commercial  sulphite  digester, ^  which  has  a  capacity 
of  about  85  pounds  of  oven-dry  chips , 

The  cooking  acid  used  was  adjusted  so  as  to  approxi- 
mate 5.50  per  cent  total  sulphur  dioxide,  of  which  about  1.30 
per  cent  was  present  as  combined  sulphur  dioxide. 

In  order  to  counteract  the  excessive  condensation  of 
steam  caused  by  the  abnormal  radiation  from  the  small-sized 
equipment,  both  direct  a.nd  indirect  steam  heating  were  used 
for  the  digester.   The  steam  for  direct  heating  was  so  regu- 
lated in  amount  that  the  dilution  effect  of  the  condensed 
steam  was  similar  to  that  obtained  in  commercial  "quick-cook" 
practice.   This  variable  is  under  control  by  observation  and 
test  of  the  relief  liquor. 

Temperature  and  pressure  Schedule 

The  rate  of  temperature  increase  and  the  pressure 
schedule  for  each  cook  are  shown  in  Figures  1,  2,  3  and  4.   A 


4 

—A  complete  description  of  the  equipment  may  be  found  in  U.S. 

Department  of  Agriculture  Bulletin  No. 1485,  11-15  (1S27). 
R883  -4- 


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Fig.  2. — Temperature-pressure  chart  fcr  2.0  hours 
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Fig.  3*  —  Temperature-pressure  chart  for  2.5  hours 
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R883 


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Hours 


Fig.  k-.  —  Temperature-pressure  chart  for  3o  and  5 
hours  penetration  periods. 


R883 


maximum  pressure  of  75  pounds  was  used  throughout,  followed 
by  a  uniform  relief  period  at  the  completion  of  the  cook. 

Yield  determinations  were  made  by  screening  the 
pulp  over  a  diaphragm  screen  having  0.012-inch  slots.   Samples 
for  moisture  determination  were  taken  at  regular  intervals  as 
the  pulp  was  run  over  the  experimental  wet  machine,   Kni 
the  total  weight  of  wet  pulp  and  the  moisture  content,  the 
oven-dry  weight  of  pulp  was  calculated.   Screenings  were  oven- 
dried  and  weighed. 

The  bleach  requirement  of  the  various  pulps  were 
determined  by  treating  a  sample  equivalent  to  25  grams  (oven- 
dry  weight)  of  pulp,  in  a  water  suspension  at  a  constant  con- 
sistence of  1.5  per  cent,  with  varying  ratios  of  bleach  powder 
to  pulp.   Temperature  was  maintained  constant  at  35  degrees  C. 
and  the  stirring  was  continuous  until  the  chemical  was  ex- 
hausted.  The  pulp  was  then  thoroughly  washed,  made  up  into 
hand  sheets,  and  dried.   After  careful  pressing  of  the  dry 
sheets,  color  measurements  were  made  on  the  samples  by  the 
use  of  the  Hess-Ives  tint  photometer.   Samples  of  bleached 
Burgess  sulphite  pulp  were  used  as  a  standard  for  comparison. 
The  bleach  requirement  was  taken  a.s  the  amount  of  bleach  powder 
needed  to  bleach  the  sample  of  pulp  to  the  same  degree  of  white- 
ness as  that  of  the  standard. 

The  ability  of  the  pulps  to  develop  fiber  strength 
was  determined  by  the  ball-mill  method  as  standardized  at  this 
Laboratory.  -Test  sheets  were  prepared  from  the  unbleached 
pulps  after  20,  40,  60,  and  80  minutes  milling.   Bursting- 
strength,  folding-endurance,  and  tearing-st     th  tests  were 
made  for  each  milling  period  and  each  cook. 


Tabulation  of  Results 


The  pulping  conditions,  pulp  yields,  and  bleach 
requirements  are  presented  in  detail  in  Table  2. 

The  data  on  the  development  of  fiber  strength  by 
ball-milling  the  unbleached  pulos  are  presented  in  detail  in 
Table  3. 


Discussion  of  Results 


As  previously  stated,  the  main  emphasis  in  the  ex- 
perimental work  performed  thus  far  has  been  placed  on  the 


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RSS3 


-7- 


investigation  of  the  penetration  period  by  varying  its  time 
and  the  temperature  employed;  the  effects  of  such  variation 
were  gauged  by  the  yield  of  screened  pulp,  the  amount  of 
screenings,  and  the  strength  qualities  and  bleach  requirement 
of  the  pulps  produced. 

Effect  Upon  Yields 

Increasing  the  temperature  at  1.5  hours  time  had  no 
effect  on  the  total  yield  of  pulp  and  the  percentage  of 
screenings.   The  total  yield  of  pulp  for  each  of  the  thr 
cooks  was  low  and  the  screenings  were  comparatively  hi     On 
the  other  hand,  increasing  the  temperature  at  2.0  and       >ars 
had  a  decided  effect  on  the  total  yield  of  pulp.   The  yie  L 
of  total  pulp  and  of  screened  pulp  decreased  with  no  change 
in  the  percentage  of  screenings. 

Maintaining  the  temperature  constant  at  110  degrees 
C.  and  increasing  the  time  of  the  penetration  period  resulted 
in  an  increase  in  the  total  yield  of  pulp  and  a  decrease  in 
screenin  s.   For  the  5-hour  period  the  total  yield  of  pulp 
was  slightly  lower  than  had  been  expected.   Even  though  the 
total  yield  was  low  the  screened  pulp  was  2  per  cent  higher 
and  the  screenings  were  2  per  cent  lower  than  the  corresponding 
yields  obtained  for  the  experiment  in  which  1.5  hours  was  used 
for  penetration. 

Increasing  the  time  at  115  degrees  C.  and  120  degrees 
C,  respectively,  resulted  in  an  increase  of  total  yield  of 
pulp  and  a  decrease  in  screenings.   Pulping  experiments  having 
longer  periods  of  penetration  than  2.5  hours  were  not  made  at 
the  higher  temperatures. 

The  pulps  of  highest  yields  and  lowest  percentage  of 
screenings  were  obtained  from  the  experiments  in  which  a  2- 
hour  or  longer  period  was  used  for  penetration.   The  maximum 
temperature  of  penetration  seems  not  to  affect  the  yield 
properties  very  markedly,  except  when  short  penetration  periods 
are  used.   A  temperature  of  115  degrees  C.  may  be  used  as  the 
maximum  temperature  with  good  results. 

Effect  Upon  Bleachability 

An  increase  in  the  maximum  temperature  for  any 
penetration  period  of  constant  length  results  in  a  loweri. 
of  the  bleach  requirement  of  the  pulps. 


R883  -8- 


Holding  the  temperature  constant  at  110  degrees  C. 
and  increasing  the  time  of  penetration  from  1.5  to  2.5  hours 
had  no  effect  on  the  bleach  requirement  of  the  pulps.   A 
further  increase  in  time  of  penetration  from  2.5  to  5  hours 
lowered  the  bleach  requirement  from  34+  per  cent  to  26  per 
cent. 

A  slight  increase  in  the  bleach  requirement  resulted 
as  the  temperature  was  held  constant  at  115  degrees  C.  and  120 
degrees  C. ,  and  the  time  increased.   For  all  practical  pur- 
poses this  increase  in  bleach  requirement  may  be  neglected, 
since  it  was  only  2  per  cent. 

The  optimum  conditions  in  regard  to  bleach  require- 
ment of  the  pulps  are  obtained  by  utilizing  the  higher 
peratures  (115  degrees  C.  and  120  degrees  C.)  for  periods  cf 
1.5,  2.0,  and  2.5  hours,  or  by  extending  the  time  at  the  lov 
temperature  (110  degrees  C.)  to  at  least  5  hours. 

Effect  Upon  Bursting  Strength 

The  bursting-strength  factor  of  pulps  from  experi- 
ments in  which  a  temperature  in  excess  of  110  degrees  C.  is 
reached  in  1-1  / 2  hours  time  are  somewhat  lov;.   Increasing  the 
temperature  at  longer  periods  apparently  has  no  effect  on  the 
bursting  properties  of  the  pulps.   The  maximums  are  reached 
after  60  minutes  refining  in  the  ball  mill. 

With  an  increase  in  time,  and  the  temperature  con- 
stant at  110  degrees  C. ,  no  increase  in  the  maximum  bursting- 
strength  factor  was  noted.   On  the  other  hand,  an  increase 
in  bursting  properties  resulted  upon  increasing  the  time  at 
temperatures  of  115  degrees  C.  and  120  degrees  C. 

The  conditions  tha.t  yield  pulps  with  maximum  burst- 
ing properties  indicate  that  2  to  2-1/2  hours  time  is  essen- 
tial for  the  increase  to  a  temperature  of  110  degrees  C.  to 
120  degrees  C. 

Effect  on  Folding  Properties  and  Tearing  Strength 

There  are  no  general  trends  in  the  folding-endurance 
or  tearing-strength  factors  of  the  pulps  with  increased  tem- 
perature or  time. 


R333 


A 

: '    o  - 


Summary 

The  results  obtained  in  this  series  of  pulping 
experimenta  may  be  summarized  as  follows: 

The  yields  of  pulp  decrease  with  an  increase  in 
temperature  during  the  penetration  period,  the  amount  of 
screenings  remaining  practically  constant. 

With  an  increase  in  the  time  of  the  penetration 
period  the  yields  of  pulp  increase  for  temperatures  of  115 
degrees  C.  and  120  degrees  C. ,  but  fail  to  do  so  for  110 
degrees  C. 

A  decrease  in  screenings  is  obtained  by  increasing 
the  time  of  the  penetration  period. 

A  definite  reduction  in  the  bleach  requirement  of 
the  pulps  is  obtained  with  an  increase  in  temperature  of  the 
penetration  period. 

The  bursting  strength  factor  decreases  as  the  tem- 
perature of  the  penetration  period  is  increased  from  110 
degrees  C.  to  115  degrees  C.  and  120  degrees  C,  at  l-l/2 
hours  ti...  . 

As  the  time  of  penetration  is  increased  for  115 
degrees  C.  and  120  degrees  C. ,  respectively,  the  bursting 
strength  factor  increases. 


R883  -10- 


UNIVERSITY  OF  FLORIDA 


3  1262  08928  6172 


